The application of acoustic lenses which, in terms of accurate undistorted imaging and focusing, is most exacting is that of nondestructive imaging or testing. For "real-time" ultrasonic imaging of organs in a living organism, e.g., such as a heart in a living human body, it is important to be able to sequester all acoustic waves containing image information and produce the "acoustic image" with an absolute minimum of distortion and loss of acoustic energy. The composite lens assembly described here is specifically designed and constructed for such use and, therefore, the description is made in connection with this most demanding application of acoustic lenses. However, it will be particularly understood that the structures and principles are applicable in many other uses of acoustic imaging and focusing. For example, a good application is for focusing acoustic waves generated by a transducer.
A major loss of acoustic energy which would otherwise be available for acoustic imaging is caused by mode conversion at the interface between a liquid transmitting the acoustic waves and a solid, such as a lens element. Specifically, we are concerned with a conversion of an incident compressional wave, which can be translated to a meaningful and useful acoustic image, to a shear wave which in most systems is useless and in some measure is counterproductive. Because of the balance of shear strain at the liquid/solid boundary, there is no mode conversion when the incident compressional wave is normal to the surface of the solid encountered. However, as the angle of incidence is increased, more of the compressional wave is converted to shear wave energy and, indeed, there is an angle (called the critical angle) at which an incident compressional wave is substantially totally transformed into a shear wave.
Thus, the acoustic lens designer is confronted with the problem of producing a lens element or elements having a sufficiently small (short) radius of curvature to provide the proper imaging and focusing action without presenting such a steep liquid interface as to convert an appreciable amount of the incident compressional wave energy to energy in the form of shear waves.
In regard to acoustic lens design, there is a close analogy between reflection and refraction of optical and acoustic wave fronts at boundaries separating regions of different refractive index; therefore, acoustic lenses and reflectors are designed in accordance with the same procedures used in optics. With few exceptions, the analogy between acoustics and optics extends to all scalar propagation phenomena. As might be expected, there exists for an acoustical lens or focusing reflector an image-plane/object-plane relationship that is identical to that found in optics. Specifically, a spatial pattern of acoustic pressure in a plane in front of an acoustic lens (and propagating toward it) induces in a conjugate plane of the lens a diffraction and aberration limited replica of itself.
In view of the analogy between acoustics and optics, the general theory of lens design (and, in fact, practical lens design) is well understood for ultrasonic lenses. Therefore, an extended explanation of principles of lens design is not given here. Only those principles of acoustic lens design which constitute part of this invention and which are not found in the tutorials on ultrasonic lenses is emphasized. For the principles of general acoustic lens design, one may refer to texts such as SONICS by Heter and Bolt, John Wiley and Sons, publishers, 1955, p. 265, "Sound Focussing Lenses and Waveguides," T. Tarnoczy, ULTRASONICS, July-September 1964-1965, pp. 115-127, and "The Aberrational Characteristics of Acoustic Lenses," B. D. Tartakovskii, SOVIET PHYSICS-ACOUSTICS Vol. 8, No. 3, January-March 1963.